OVERVIEW OF PNEUMONIA
Infections of the lower respiratory tract may involve the airways, lung parenchyma, or pleural space. Pneumonia is an infection of the gas exchanging units of the lung, most commonly caused by bacteria, but occasionally by viruses, fungi, parasites, or other infectious agents. In immunocompetent individuals, pneumonia is characterized by a brisk filling of the alveolar space with inflammatory cells and fluid. If the alveolar infection involves an entire anatomic lobe of the lung, it is termed lobar pneumonia, and some episodes may lead to multilobar illness and more severe clinical manifestations. When the alveolar process occurs in a distribution that is patchy and adjacent to bronchi, without filling an entire lobe, it is termed bronchopneumonia.
Based on clinical presentation, pneumonias have also been classified as being typical or atypical. The
typical pneumonia syndrome is characterized by a sudden onset of high fever,
shaking chills, pleuritic chest pain, and productive cough, and it can be
expected only if the patient has an intact immune response system and if the
infection is caused by a bacterial pathogen such as Streptococcus pneumoniae,
Haemophilus influenzae, Klebsiella pneumoniae, Staphylococcus
aureus, aerobic gram-negative bacilli, or anaerobes. If a patient is infected by one of these
organisms but has an impaired immune response, the classic pneumonia symptoms
may be absent, as can be the case in elderly and debilitated patients. The
atypical pneumonia syndrome, characterized by preceding upper respiratory
symptoms, fever without chills, nonproductive cough, headache, myalgias, and
mild leukocytosis, is often the result of infection with viruses, Mycoplasma
pneumoniae, Chlamydophila pneumoniae, Legionella organisms,
and other unusual infectious agents (as in psittacosis and Q fever). In
clinical practice, it is often very difficult to use clinical features to
predict the microbial cause of pneumonia.
When a parenchymal lung infection leads to break-down
of lung tissue, it may cause tissue necrosis and cavity formation, and this
type of infection is termed a lung abscess. These infections usually
result when a patient aspirates a highly virulent pathogen into the lung in the
absence of effective clearance mechanisms; the etiologic agents include S.
aureus, K. pneumoniae, Escherichia coli, and Pseudomonas
aeruginosa. Empyema is an infection of the pleural space characterized by
grossly purulent material that is usually caused by extension of parenchymal
infection outside the lung; it is caused by anaerobes, gram-negative bacilli, S.
aureus, and occasionally tuberculosis (TB).
Another classification system that is applied to
pneumonia relates to the place of origin of the infection. When the infection
occurs in patients who are living in the community, it is termed community-acquired
pneumonia (CAP), although it is called nosocomial pneumonia, or hospital–acquired
pneumonia (HAP) if it arises in a patient who is already in the hospital.
When HAP develops in a patient who has been on mechanical ventilation for at
least 48 hours, it is termed ventilator-associated pneumonia (VAP). The
distinction between CAP and HAP is becoming increasingly blurred because of the
complexity of patients who reside out of the hospital. When pneumonia develops
in patients who come from a nursing home, in those receiving chronic
hemodialysis, and in those admitted to the hospital in the past 3 months, it is
termed health care–associated pneumonia (HCAP). Because of their contact
with the health care environment, these patients may already be colonized with
multidrug-resistant organisms when they arrive at the hospital. Thus, the
relationship between bacteriology and the place of origin of infection is a
reflection of several factors, including the comorbid illnesses present in the
patient, their host-defense status, and their environmental exposure to specific
pathogens.
Patients who develop pneumonia while receiving
immunosuppressive therapy or who have an abnormal immune system are referred to as compromised hosts, and the infectious
possibilities vary with the localization of the immune defect. In recent years,
particularly with the application of immunosuppressive therapy for a variety of
illnesses, with the emergence of AIDS, and with an increasing number of
institutionalized elderly individuals, TB, fungal, and parasitic lung
infections have reemerged as important and common infections.
